At \(t=98\)  ps, the original vortex lines have experienced a large bending in order to hit perpendicularly the droplet surface.
As in the vortex-free case, two protrusions appear on the droplet surface, whose subsequent collapse
in the body of the droplet launches a series of vortex rings.
The appearance of surface protrusions is frequent in classical droplets collisions.[1]}, [2]}, [3]}, [4]}
What makes protrusions unique in the case of \(^4\) He droplets is that they act as nucleation sites of quantized vortex rings.
Surface protrusions have also been identified in head-on collisions of self-bound quantum droplets,[5]}, [6]} but
no vortex ring nucleation has been reported in these studies. For these droplets made of ultradilute Bose-gase mixtures, a minimum
number of atoms is needed to develop any binding, and the surface tension is very small.
As a result, once created the
protrusions evaporate almost immediately instead of being re-adsorbed within
the merging droplet, hindering the nucleation of vortex rings.
The helium-drop mechanism should also work for colliding classical droplets, but we are not aware that
non-quantized vortex ring nucleations have been reported in droplets collisions.
